1. Field of the Invention
The present invention relates to a generator set having an inverter and, more particularly, to a generator set having an inverter which can be operated at higher efficiency throughout a wider range of revolutions even if the output of an inverter is changed.
2. Description of the Related Art
It is well-known that an engine operated generator for use as an alternating current power source is equipped with an inverter for stabilizing the frequency of its output. FIG. 6 is a block diagram showing an example of a generator set having an inverter. A three-phase alternating current output generated by a generator 100 is converted into direct current form by a rectifier 110 including thyristors and then transferred via a capacitor smoothing unit 120 to an inverter 130. The inverter 130 converts the direct current input into an alternating current output of a commercial frequency. Some generator sets having an inverter with engines are disclosed in Japanese Patent Laid-open Publications (Heisei)11-308896 and 6-121597. As its output frequency hardly depends on the revolution of an engine, the generator set having an inverter can be operated by controlling the revolution of the engine to produce a desired output, that is, an output corresponding to a load.
The generator set having an inverter capable of determining the output of its inverter corresponding to the load, however, exhibits a disadvantage when its rectifying unit is controlled by switching so that the direct current input of the inverter stays not higher than a target voltage level. In case that the generator is a relatively small sized magnetic generator, it may fail to supply the inverter with a power enough to produce a peak of the output. For example, when the inverter is energized by the generator of 3 KW output for producing an output of 2 KVA, its peak output will be 4 KVA. A shortage in the power output of the generator is hence made up for by discharging a capacitance of the capacitor smoothing unit. However, if the capacitance is small, it can hardly make up for the shortage. The direct current voltage to the inverter is controlled so as to not exceed a target level. Because the direct current voltage to the inverter stays not higher than the target level and the capacitance in the capacitor smoothing unit remains small, a compensated level of the direct current voltage will hardly reach the target level, on average.
As a result, the output of the generator at a lower range of voltage may be used even if its level is not enough. In particular, as the generator at a higher revolution fails to produce a desired high level of power output, its operational efficiency will be declined.
This will be explained in more detail.
FIG. 7 illustrates different profiles of the power output of the generator of a magnetic type where the speed is a parameter. As shown, the curves A, B, and C represent the relationship between the current and the voltage output of the generator at H, M, and L (H greater than M greater than L) of the speed, respectively. Denoted by Ap, Bp, and Cp are three outputs of the generator at the curves A, B, and C, respectively. For having the output of the inverter adjusted to match a load, the direct current voltage at the input of the inverter is controlled to maintain at a target voltage level V. It is clearly apparent from the diagram that when the direct current voltage at the input of the inverter is maintained at the target voltage level V, the generator at L, M, and H of the speed can produce different outputs denoted by p, q, and r assigned on the curves Ap, Bp, and Cp, respectively.
However, if the average voltage at the input of the inverter remains low, as denoted by Vxe2x80x2, but does not reach the target voltage level V, the output of the generator may stay at pxe2x80x2, qxe2x80x2, and rxe2x80x2 on the curves Ap, Bp, and Cp.
It is an object of the present invention to provide a generator set having an inverter which eliminates the foregoing drawback and when the power supply is short, can use a higher range of voltage output of its generator and thus be improved in the operational efficiency.
As a first feature of the present invention, a generator set having a converter composed of semiconductor rectifying devices for rectifying the power output of a magnetic generator, a capacitor smoothing means for smoothing a direct current output of the converter, and an inverter for converting an output of the capacitor smoothing means into an alternating current form at a particular frequency is provided comprising: a semiconductor rectifying device driving means for controlling the conduction of the semiconductor rectifying devices to maintain the smoothed voltage output of the capacitor smoothing means at a target voltage level; an average calculating means for calculating an average of the smoothed voltage output; and a target voltage level adjusting means for increasing the target voltage level when the average is smaller than the target voltage level by a specific amount. As a second feature of the present invention, the average of the smoothed voltage output may be calculated at every output period of the inverter.
These features allow the target voltage level of the smoothed voltage output of the capacitor smoothing means to be increased when its average is reduced by a specific amount from the target voltage level. Then, the smoothed voltage output is modified to the adjusted target voltage level.
As a third feature of the present invention, the generator set having an inverter may further comprise: a conduction rate detecting means for detecting the conduction rate of the semiconductor rectifying devices; and an engine speed controlling means for controlling the speed of an engine to drive the generator so that the conduction rate is converged on a predetermined target rate, wherein the controlling of the speed of the engine is implemented by adjusting the supply of fuel to the engine.
The third feature allows the speed of the engine to be varied throughout a wider range to control the conduction of the semiconductor rectifying devices to a desired rate.
As a fourth feature of the present invention, the engine speed controlling means may be arranged to decrease the revolution of the engine when the deviation of the conduction rate from the target rate is positive and increase the same when negative. As a fifth feature of the present invention, a rate of change of the speed of the engine is greater at a increase than at a decrease thereof.